Micro-optic materials for projecting synthetic images generally comprise (a) a light-transmitting polymeric substrate, (b) an arrangement of micro-sized image icons located on or within the polymeric substrate, and (c) an arrangement of focusing elements (e.g., microlenses). The image icon and focusing element arrangements are configured such that when the arrangement of image icons is viewed through the arrangement of focusing elements, one or more synthetic images are projected. These projected images may show a number of different optical effects. Material constructions capable of presenting such effects are described in U.S. Pat. No. 7,333,268 to Steenblik et al., U.S. Pat. No. 7,468,842 to Steenblik et al., U.S. Pat. No. 7,738,175 to Steenblik et al., International Patent Publication No. WO 2005/106601 A2 to Commander et al., International Patent Publication No. WO 2007/076952 A2 to Kaule et al.; International Patent Publication No. WO 2009/000527 to Kaule et al.; International Patent Publication No. WO 2009/000528 to Kaule et al.; International Patent Publication No. WO 2009/000529 to Kaule et al.; and International Patent Publication No. WO 2009/000530 to Kaule.
These optically variable materials may be used as security devices for authentication of banknotes, other secure documents, and products. For banknotes and other secure documents, such optically variable materials are typically used in the form of a strip, thread, patch, or overlay and either partially embedded within the banknote or other secure document, or applied to a surface thereof. These materials may also be used as a standalone product that serves as a substrate for a subsequent printing or personalization process.
The present inventors have determined that these optically variable materials possess a certain degree of optical sensitivity related to susceptibility on the part of the focusing element arrangement (e.g., focusing element array) to soiling, physical ablation (e.g., scratching), and to disturbances in focal properties whenever a disrupting material is brought into contact with a surface of the array. Disrupting materials causing such a disturbance in focal properties include adhesive coated substrates (e.g., tapes), liquids, or other materials having a refractive index that is different from that of air. In particular, the synthetic image or images projected by these materials tend to disappear, defocus, or blur when such a disrupting material is applied to the focusing element array surface, the disrupting material causing an undesirable alteration in the angle of refraction at the array surface.
The present invention addresses this issue by providing a system for projecting one or more synthetic optical images, which demonstrates improved resistance to optically degrading external effects. The inventive system basically comprises:                (a) one or more arrangements of image icons; and        (b) one or more partially or totally embedded arrangements of image icon focusing elements,        wherein the one or more arrangements of image icon focusing elements is disposed relative to the one or more arrangements of image icons such that at least a portion of the image icon focusing elements forms at least one synthetic image of at least a portion of the image icons.        
In an exemplary embodiment, the one or more arrangements of image icon focusing elements are refractive focusing elements (e.g., microlenses). The refractive index from an outer surface of this exemplary system to refracting interfaces is varied between a first and a second refractive index, the first refractive index being substantially or measurably different than the second refractive index.
In this exemplary embodiment, the arrangement(s) of focusing elements lie between a viewer's eyes and the arrangement(s) of image icons, with variation of the refractive index achieved using a material (hereinafter referred to as “the second material”) that either (i) fills interstitial spaces between at least a portion of the image icon focusing elements and/or covers these focusing elements, forming a distinct interface with the material used to form the focusing elements (hereinafter referred to as “the first material”), or (ii) diffuses into the first material thereby forming a gradient interface with the first material. The second material may either partially or totally embed the arrangement(s) of focusing elements or may encapsulate the inventive system. More preferably, the second material either forms an outer boundary (or layer) of the arrangement(s) of image icon focusing elements (total embedment of the arrangement(s) of focusing elements), or forms an outer boundary (or layer) of both the arrangement(s) of image icon focusing elements and the arrangement(s) of image icons (total encapsulation of the system).
The phrase “substantially or measurably different”, as used herein, means a difference in refractive index (e.g., between the first and second materials) that causes the focal length(s) of the focusing elements to change at least about 0.1 micron.
The focal length(s) of the focusing elements in the inventive system is locked in place by ensuring that interfaces (e.g., refracting interfaces) responsible for focus are embedded within the system. In other words, no other transparent materials or layers brought into contact with the inventive system will serve to materially alter the focal length(s), or the optical acuity of the synthetic image(s) formed by this system.
By way of the present invention, the inventors have found that in addition to providing the system with improved resistance to optically degrading external effects, the use of a material having a substantially or measurably different refractive index (e.g., the second material) over the image icon focusing elements may increase the F number of the focusing elements so as to cause exaggerated optical effects. For example, upon tilting the inventive system, synthetic images may appear deeper or further above the system, or may appear to move faster, depending on the desired optical effect.
In one preferred embodiment, the system comprises: (a) an array of image icons; (b) an array of image icon focusing elements formed from a first material having a refractive index (n1); and (c) a second material having a different refractive index (n2) that fills interstitial spaces between and/or covers the focusing elements, a distinct interface being formed between the first and second materials. In this preferred embodiment, the second material, which may totally embed the focusing element array by forming an outer boundary (or layer) of the array, may also be used to cover or embed the array of image icons, thereby encapsulating the system.
When the refractive index of the first material (n1) is greater than the refractive index of the second material (n2) [n1>n2], the focusing elements in this preferred embodiment are converging (e.g., convex) lenses. Conversely, when the refractive index of the first material (n1) is less than the refractive index of the second material (n2) [n1<n2], the focusing elements in this preferred embodiment are diverging (e.g., concave) lenses.
The embodiment in which the second material totally embeds the focusing element array may be used in the form of, for example, a security strip, thread, patch, or overlay and mounted to a surface of, or at least partially embedded within a fibrous or non-fibrous sheet material (e.g., banknote, passport, identification or ID card, credit card, label), or commercial product (e.g., optical disks, CDs, DVDs, packages of medical drugs), etc., for authentication purposes. This embodiment may also be used in the form of a standalone product (e.g., a substrate for subsequent printing or personalization), or in the form of a non-fibrous sheet material for use in making, for example, banknotes, passports, and the like. As will be readily appreciated by those skilled in the art, the visual effects offered by the inventive optical system serve to greatly increase the counterfeit resistance of these materials.
The embodiment in which the second material totally encapsulates the inventive system by forming an outer boundary (or layer) of both the array of image icon focusing elements and the array of image icons may be used as described above, or may adopt a thicker, more robust form for use as, for example, a base platform for an ID card, high value or other security document.
In another preferred embodiment, no distinct interface is formed between the array of focusing elements and the second material. Instead, the system comprises: (a) an array of image icons; and (b) an array of image icon focusing elements (e.g., GRIN lenses) formed from a first material having a refractive index (n1) and a second material having a different refractive index (n2), the second material diffusing into the first material thereby forming a gradient interface with the first material. The gradient interface acts as a focusing element, the refractive index changing spatially between, for example, outer boundaries of the second and first materials. In this preferred embodiment, the second material serves to totally embed the array of focusing elements and may also be used to cover or embed the array of image icons. Contemplated uses for this exemplary embodiment include those uses identified above.
The present invention further provides sheet materials and base platforms that are made from or employ the inventive optical system, as well as documents made from these materials. The term “documents”, as used herein designates documents of any kind having financial value, such as banknotes or currency, bonds, checks, traveler's checks, lottery tickets, postage stamps, stock certificates, title deeds and the like, or identity documents, such as passports, ID cards, driving licenses and the like, or non-secure documents, such as labels. The inventive optical system is also contemplated for use with consumer goods as well as bags or packaging used with consumer goods.
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and accompanying drawings. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.